Racial disparities in colorectal cancer (CRC) incidence persist for blacks and whites. Given the previous work of our team documenting racial differences in the gut microbiota of blacks and whites and the evidence supporting the interaction between diet and the gut microbiota as a risk factor for colorectal cancer, the study of how various diets affect the structure and function of the gut microbiota across racial groups is warranted. Previous research has shown that the gut microbiota can be rapidly altered by changes in diet. For example, consumption of an extremely high fiber (>50 grams) diet has produced changes in the gut microbiota that are believed to reduce cancer risk. The Dietary Approaches to Stop Hypertension (DASH) diet, rich in fruits, vegetables, whole grains and low-fat dairy, is commonly recommended for heart health and has been shown to lower blood pressure and produce weight loss. However, to our knowledge, the effect of the DASH diet on the gut microbiota has not been studied. Because the DASH diet provides substantial fiber, we hypothesize that consumption of the DASH diet will lead to improvements in the gut microbiota of non-Hispanic black and white adults. In this proposal, we plan to investigate our hypothesis by recruiting a generally healthy sample of 112 black and white adults from Birmingham, AL to participate in a 28-day randomized, controlled feeding study. Participants will be randomized to receive either the DASH diet or a standard American diet. All meals will be provided by the study. Fecal samples will be collected at multiple time points before, during, and after the dietary intervention and will be analyzed using PCR to amplify the V4 region of the 16S rRNA gene and to sequence bases using the MiSeq platform. Sequenced data will then be analyzed using QIIME. We hypothesize that participants receiving the DASH diet will have a greater increase in alpha diversity and greater changes in abundances of CRC-associated microbes than participants receiving the standard American diet. We will also evaluate functional-level markers including bile acid and short chain fatty acid (SCFA) production and inflammatory markers. If our hypothesis is supported, we expect to see reduced production of secondary bile acids (e.g., deoxycholic acid), greater SCFA production (e.g, butyrate), and reduction in gut and systemic inflammation (e.g, calprotectin, IL-6) among participants receiving the DASH diet compared to the standard American diet. Our findings will provide preliminary evidence for the DASH diet as an approach for cultivating a healthier gut microbiota across racially diverse populations. These findings can impact clinical, translational, and population-level approaches for modification of the gut microbiota to reduce risk of chronic diseases like CRC.